The present invention relates to a new and improved method as well as to a new and improved construction of an apparatus for producing high density or concentration alkali such as caustic soda (NaOH) and caustic potash (KOH) by evaporating a caustic alkali solution at elevated temperatures. The term "high-density or concentration alkali" within the context of the present disclosure is understood to relate to concentrations greater than, for example, 98% NaOH or 94% KOH, as the case may be.
A typical prior art apparatus used for producing high density or concentration caustic soda is shown in FIG. 1.
In this figure, reference numerals 1 and 2 denote evaporators, reference numerals 3 and 4 denote heat exchangers constituted by a multiplicity of tubes or pipes connected to the bottom section of the respective evaporators 1 and 2, reference numberal 5 denotes a heat transfer medium heater, reference numeral 6 denotes a flaker, and refernece numeral 7 denotes a condenser.
In this apparatus, a caustic soda solution having a density corresponding to a caustic soda concentration of 50% and having a temperature of 80.degree. C. is infed through a pipe 8 into the tubes or pipes of the heat exchanger 3 connected to the bottom section of evaporator 1. The caustic soda solution is heated to 98.degree. to 99.degree. C. within the heat exchanger 3 by means of a 100.degree. C. heat source which is formed by a vapor or steam introduced into the heat exchanger 3 through a pipe 9 from the upper end of the evaporator 2. Heating reduces the water content so that the caustic soda solution is now concentrated to a density corresponding to a caustic soda concentration of about 60%.
Reference numeral 10 denotes a discharge pipe through which the condensate is discharged after heat exchange from the heat exchanger 3.
The 60% caustic soda solution which accumulates in the bottom section of the evaporator 1, is transferred to the evaporator 2 through a pipe 11 leading from the bottom section of the evaporator 1 and to the tubes or pipes of the heat exchanger 4 connected to the bottom section of the evaporator 2. The caustic soda solution is heated in the heat exchanger 4 by molten salt which is supplied thereto from the heat transfer medium heater 5 through a pipe 12 and passed through the heat exchanger 4. As a result, there is obtained a 98.5% caustic soda solution at a temperature of about 400.degree. C. Reference numeral 13 denotes a pipe through which the molten salt is discharged after the heat exchange in the heat exchanger 4.
The vapor or steam withdrawn at the upper end of the evaporator 1 is cooled by the condenser 7 and the condensate is discharged through a pipe 14. Any non-condensable gas is withdrawn through a pipe 15 and thereafter through a pipe 17 under the action of a steam ejector 16. Reference numeral 18 denotes a pipe for supplying steam and reference numerals 19 and 20 respectively denote pipes for supplying and discharging water.
The caustic soda solution having a density corresponding to a caustic soda concentration of 98.5% is collected in the bottom section of the evaporator 2 and is infed into the flaker 6 through a pipe 21 leading from the bottom section of the evaporator 2. The flaker 6 contains a rotary drum 6a and water is supplied to its inside so as to cool the caustic soda solution to flakes 22 of high density or concentration caustic soda.
In this prior art arrangement the flaker 6 is at atmospheric pressure and therefore it is difficult to maintain a higher pressure in the evaporator 2. Therefore the internal pressure in the evaporator 2 of the conventional apparatus must be maintained at atmospheric pressure. However, if higher pressure would be tried to be maintained inside the evaporator 2, it would be necessary to provide suitable pressure-reducing means in the pipe 21 in order to thereby prevent such higher internal pressure which would prevail in the evaporator 2, from being relieved into the flaker 6. Such pressure-reducing means might be constituted by a valve, an orifice or a restriction. Unfortunately, however, no material is available which is sufficiently corrosion resistant for effecting restriction of the flow of high density caustic soda at high temperature. The best material now available is pure nickel but even this quickly corrodes and is eroded and soon becomes unusable when exposed to a suddenly expanding mixture of vapor or steam and high density caustic soda at high temperature. Therefore, the internal pressure of the evaporator 2 which is directly connected to the flaker 6 through the pipe 21, usually is maintained at atmospheric pressure in the prior art.
In order to attain a 99% caustic soda solution in the evaporator 2, the molten salt is supplied as the heat source or heat transfer medium to the heat exchanger 4 and heats the interior of the evaporator 2 to 400.degree. C. As a consequence, steam of 400.degree. C. is infed into the pipe 9. Due to the high energy of this steam, such steam, if directly supplied to the heat exchanger 3, would concentrate the 50% caustic soda solution of about 80.degree. C. as supplied through the pipe 8, up to about 65%, provided that adequate vapor pressure is achieved. In this conventional arrangement, however, the vapor or steam can only be used as a heat source of the relatively low saturation temperature of about 100.degree. C. so that the evaporator 1 produces a caustic soda solution of about 60% only. If all the vapor or steam from the evaporator 2 would be introduced into the heat exchanger 3, the internal pressure in the evaporator 2 would rise and the excess pressure would be undesirably relieved into the flaker 6.
For these reasons, in the conventional apparatus shown in FIG. 1, not all the vapor or steam fed from the evaporator 2 into the pipe 9 is introduced into the heat exchanger 3, but some of the vapor or steam is discharged into the atmosphere through an intermediate section of the pipe 9 which results in a highly undesirable waste of energy.